A major trend in the electronics industry is to make products smaller, lighter and faster while maintaining or improving their functionality. One of the key technologies that is enabling the creation of more and more compact products is electronic packaging and assembly technology. Electronic packaging and assembly constitutes the materials and processes required to interconnect a semiconductor chip (IC chip) to other electronic or electrical components. Besides the semiconductor chip, various materials can be incorporated into an electronic package, such as flexible circuitry (metal circuitry on polyimide or other polymer films), metal stiffeners, electrically conductive layers, electrically insulating layers, and heat sinks. Often adhesives are used to adhere these various substrates together and to adhere together multiple layers of circuitry on substrates to form multilayered electronic structures having increased wiring density such as may be required for mulitchip modules. Covercoats and encapsulants are needed to protect the circuitry from harsh environments that may be seen in the products' application. Electrical connection of these packages to printed circuit boards or other electronic components requires that the package undergoes solder reflow, which can expose the package to temperatures of 220.degree. C. for tens of seconds to minutes. This process has proved to be quite demanding on adhesives and cover coats, causing product failures such as voiding and/or delaminating from the various substrates. "Popcorning" is the generic term coined for these failures. The presence of moisture in the package followed by rapid heating to solder reflow temperatures promotes "popcorning". Moisture is sorbed by many organic substrates such as polyimide films and organic adhesives, and it can sorb to metals and inorganic components.
Joint Industry Standard, "Moisture/Reflow Sensitivity Classification for Plastic Integrated Circuit Surface Mount Devices", October 1996, J-STD-020, developed by the Electronic Industries Association's Joint Electron Device Engineering Council (JEDEC) and the Institute of Interconnecting and Packaging Electronic Circuits, is the electronics industry standard for testing the solder resistance of packages after exposure to humidity. This standard grades materials by levels and JEDEC level 1 (85.degree. C./85% R.H. for 168 hours followed by solder reflow at a peak temperature of 220.degree. C. for 10 to 40 seconds) is the most demanding test. Packages that pass JEDEC level 1 have the highest degree of solder resistance, and need not be packaged so as to protect them from ambient moisture. Failure at level 1, but passage at less-demanding levels, requires protective packaging.
The current state of the art in adhesive development allows the achievement of JEDEC level 3 (30.degree. C./60% R.H. for 192 hours followed by solder reflow at a peak temperature of 220.degree. C.) and a very limited number of adhesives claim successful performance of JEDEC level 2 (85.degree. C./60% R.H. for 168 hours followed by solder reflow at a peak temperature of 220.degree. C.). Specific polyimides can pass JEDEC level 1, but these materials require extremely high lamination temperatures of approximately 350.degree. C. Such lamination temperatures prohibit polyimide use in packages that contain materials that will decompose or be altered at these high temperatures, such as organic covercoats, solder masks, solder, etc. In cases where all materials present in the package at the time of lamination can withstand the high laminating temperatures, polyimides can be employed. However, these high temperatures make for a difficult and expensive process.
Epoxidized styrene-diene block copolymers, such as epoxidized styrene-butadiene or epoxidized styrene-isoprene block copolymers, have been described in U.S. Pat. No. 5,478,885. In some applications, the epoxidized block copolymers have been used as rubber toughening agents for commonly-used epoxy resins. Typically, the toughening agent constitutes a small percentage of the total composition.
Cured compositions comprising epoxy resins and epoxy-modified aromatic vinyl-conjugated diene block copolymers have been described in EP658603. The compositions can contain from 5 to 95 parts by weight of an epoxy resin, preferably from 20 to 80 parts by weight. When the composition contains less than S parts by weight of either component, a loss of mechanical properties is reported. Use of these materials in electronic packaging applications is not suggested.
Crosslinking epoxidized styrene-diene block copolymers through the backbone epoxy groups has been shown to produce adhesive compositions for use in, for example, pressure-sensitive adhesive tapes, labels, sealants, and coatings as disclosed in U.S. Pat. No. 5,229,464 and WO 97/30101. The adhesives can be formulated to include reactive diluents, including epoxy resins, in the amount of from 1 to 50% by weight of the total composition. Use of these materials in electronic packaging applications is not suggested.
Compositions comprising cured epoxidized styrene-diene block copolymers useful as multilayered molding materials have been described in WO 98/22531. The compositions can be cured by a wide variety of known epoxy curing agents. Compositions further comprising added polyfunctional co-reactants, in amounts ranging from 0.01 to 25 parts by weight, are also described. Epoxy resins are not disclosed as co-reactants. Use of these materials in electronic packaging applications is not suggested.
Adhesives useful in electronic packaging comprising liquid epoxy resins and an added functionalized resin have been described in EP 387066 (see also U.S. Pat. No. 5,843,251). In order to extend the shelf life of the adhesives, a micro-capsule type curing agent is required in the formulation. Functionalized styrene-diene block copolymers are among the functionalized resins named and not exemplified, wherein the functional group can be an epoxy group. The added functionalized resin can be present in the amount of from 20 to 80% by weight. The adhesives can be processed into adhesive tapes that can be used to electrically connect circuits on chips and wiring substrates. Performance of the adhesives under conditions of JEDEC Level 1 is not described.
Amine crosslinkable hot-melt adhesives useful in electronics applications, comprising a polyolefin having an epoxy group in its molecule, have been described, for example, in WO 96/33248, wherein an aromatic amine curative is required and the composition optionally comprises an epoxy resin. Polyolefins mentioned are copolymers of ethylene and a glycidyl (meth)acrylate group-containing monomer, so that the epoxy groups are pendant from the polymer backbone. Epoxy-modified aromatic vinyl-conjugated diene block copolymers are not suggested. When present, the amount of added epoxy resin is from about 5 to about 200 parts by weight per 100 parts of the polyolefin copolymer. The adhesives were used for electronic systems, but their use under conditions of JEDEC Level 1 is not suggested.